The laying of long lines of metal pipe on the floor of deep waters from a lay barge is gaining increasing commercial importance for the transport of fluids, particularly petroleum and petroleum products.
In one conventional, pipe-laying method, known as "stove-piping", a pipeline is fabricated on the deck of a lay barge by welding together individual lengths of pipe. Each length of pipe is about 40 feet long. In another conventional method, known as "the reel pipe-laying technique" for laying relatively smaller diameter pipe, a pipeline is wound on the hub of a large reel which is rotatably mounted on the deck of the lay barge. The lay barge is then moved to an offshore pipe-laying location and the pipeline is unwound from the reel. The unwinding portion of the pipeline is straightened prior to being lowered into the body of water. In both of these techniques the departure angle of the pipe relative to the barge is limited primarily due to the fixed location of the pipe handling machinery mounted on the deck of the barge.
In either of these known pipe-laying methods, the lay barge is required to have a long equipment ramp in a position fixed relative to the barge. In one procedure there are mounted on the ramp a plurality of longitudinally-spaced pipe supports, commonly known as stanchions. The position of each stanchion is adjustable within a small range to provide a desired pipe-descent trajectory which gradually slopes downwardly. A particular trajectory is selected for each diameter size of pipe, weight of pipe, pipe tension, and depth of water. In another procedure the means of supporting the pipe is what is commonly known as a "stinger", which is attached to the aft end of the lay barge at the end of the long ramp. The stinger supports the descending pipe span from the aft end of the barge to a lift-off point on the stinger. From this lift-off point the pipe span can safely sag to the seabed. The primary purpose of the stinger is to avoid excessive concentrations of curvature stress in the descending pipe span. These concentrations could buckle or permanently bend the pipeline. Various types of pipe stingers are known; some are straight and others have articulated joints.
A straight stinger is usually a long, stiff structure. It comprises two buoyant pontoons interconnected by numerous crossovers on which are mounted horizontally and vertically disposed rollers. The rollers support the pipe span as it moves from the barge's aft end to the lift-off point. However, straight stingers have several disadvantages, for example, significantly different water depths require stingers of different lengths. The changing of stingers is time-consuming. The length of a straight stinger is generally from four to six times the water depth in which the pipeline is laid. In practice, however, stingers have an upper length limit of about 600 feet. Such long stingers must be capable of withstanding high loads caused by water currents and relative movement between the stinger and the lay-barge. Stingers are therefore generally constructed from high-strength materials, for example high-strength steel.
Some of the disadvantages of a straight stinger are avoided by using a freely-articulated stinger consisting of several segments connected in series by hinge joints. While a freely-articulated stinger is designed to follow the curvature of the descending pipe span, destructive concentrations of pipe curvature stress can occur along the stinger, if the buoyancy of the segments is not correctly distributed. Wave motions and pitching motions of the lay barge tend to cause the stinger to rise and fall in high seas, thereby making a pipe which extends between any two stinger segments very vulnerable to buckling.
A semi-articulated stinger has been proposed as a compromise between a freely-articulated stinger and a straight stinger. In the semi-articulated stinger, the segments are connected with hinge joints which permit a limited degree of vertical, lateral, and torsional inter-segment movement.
Stingers usually require a significant amount of time to install, operate, and remove, especially during an impending storm or other adverse operating conditions. If a stinger fails during pipe-laying, the barge's down time can be great and the ensuing financial loss considerable.
In practicing the conventional pipe-laying methods hereinbefore described the pipeline moves over the deck of the lay barge in a substantially horizontal plane. Long ramps are required on the lay barges and frequently these ramps do not provide adequate space for inspecting, testing and, if necessary, repairing the descending pipeline. Also, workers find it difficult to maintain their equilibrium on the ramp, especially in rough sea.
Proposals have been made to solve the "stinger problem" by eliminating the stinger. But these proposals, as far as is known, have been limited to the reel pipe-laying technique for up to 12 inch pipe, as described in U.S. Pat. No. 3,641,778, assigned to the same assignee.